Muscarine and t-LHRH suppress M-current by activating an IAP-insensitive G-protein.

The control of M-current by muscarinic ACh receptors and luteinizing hormone releasing hormone (LHRH) receptors was studied in dialyzed frog sympathetic ganglion neurons. M-current was recorded in dialyzed cells without run-down or changes in its biophysical properties and could be reversibly suppressed by muscarine and teleost LHRH (t-LHRH). However, dialysis with internal solutions lacking ATP or substituting with APP(NH)P caused the loss of M-current, suggesting that dephosphorylation suppresses the activity of M-channels. M-current over-recovers after agonist addition and removal to a size 30% larger than control, as if latent channels are activated during the recovery. Dialysis of cells with the G-protein activators GTP gamma S, fluoride, and aluminum fluoride causes loss of M-current. G-protein activation by receptors was confirmed by dialysis with low concentrations of GTP gamma S in competition with GTP. This prevents the rapid loss of M-current, but addition of muscarine or t-LHRH caused irreversible loss of M-current, suggesting that both transmitter receptors do suppress M-current by activating a G-protein. Suppression of M-current was not affected by treatment with 0.1 microgram/ml pertussis toxin (IAP) for 24-48 hr. In addition, based on the lack of IAP-specific labeling of frog sympathetic neuron membrane proteins, no IAP-sensitive G-proteins are present in these cells. These results indicate that an IAP-insensitive G-protein couples muscarinic and LHRH receptors to the suppression of M-current.